This invention relates to a method of measuring and indicating the distance from a motor vehicle to an obstacle by a measurement of the capacitance existing between a capacitor plate which is fixed to the vehicle, on the one hand, and the environment, on the other hand, by a capacitance-measuring device, and also relates to a capacitive proximity sensor for motor vehicles, which sensor comprises a capacitance-measuring device for measuring the capacitance existing between at least one capacitor plate which is fixed to the motor vehicle, on the on hand, and the environment, on the other hand, and an indicator for receiving the output signal from the measuring device.
Such a proximity sensor has been described in Published German Application 2,044,790 and comprises a bridge circuit for eliminating disturbing or parasitic capacitance. Whereas certain a.c. bridge circuits can be used to eliminate even large disturbing capacitances, such a bridge circuit will be quickly and highly detuned by changes of the impedances of the branches of the bridge so that the bridge circuit is ultimately out of the range in which a compensation is possible. It must be particularly be taken into account that motor vehicles are subjected to continually changing influences from the environment and that the capacitance existing between the motor vehicle and ground will depend on the load on the vehicle. Said changes often exceed by an order of magnitude those capacitance changes which are to be measured at the capacitor plate. Parasitic resistances occur too and are continually changed. This may be due, e.g., to the influence of salt water in winter. In general, it has been found that the known bridge circuit can compensate the parasitic capacitance of a shielded line which connects the pick-up to the bridge but the continually changing resistances and capacitances outside the motor vehicle are not taken into account by that bridge circuit.
It is an object of the invention to provide a measuring method and a capacitive proximity sensor by which the disturbing parasitic capacitances occurring in practice are effectively taken into account.
In a method of the kind described first hereinbefore that object can be accomplished in accordance with the invention in that the output voltage of the capacitance-measuring device is delivered to an amplifier, which contains a degenerative feedback loop comprising a storage device and a switch so that the output signal of the amplifier is held constant during normal travel, and that the switch is opened to interrupt the degenerative feedback loop for a measurement and the switch is closed when the measurement has been completed.
The object stated is also accomplished by a provision of a proximity sensor which is of the kind described hereinbefore and in which the fixed capacitor plate consists of a plate member, particularly of a number plate, which is mounted on the body of the vehicle, or the capacitor plate is constituted by an electrically conductive layer provided on the front or rear surface of such plate member, the measuring device comprises a degenerative feedback control loop, a storage device is provided for storing the instantaneous degenerative feedback signal (deviation), and the control loop is adapted to be interrupted by a switch which is operable by the driver directly or indirectly, e.g., when the reverse gear is to be engaged.
The invention is based on the concept that the capacitor plate is constituted by a plate member, preferably by the number plate, the output of the measuring device is usually automatically held at a predetermined value by means of a degenerative feedback loop, and the last deviation corresponding to the degenerative feedback signal is stored. The control loop is interrupted by means of a switch only when the proximity sensor is actually to be used, e.g., when the reverse gear is to be engaged because the vehicle is to be parked. Deviations of the capacitance from the last value that has been measured will now be detected. Such deviations may be read directly from a measuring instrument or may be delivered to a limit comparator. When the vehicle has been parked, the switch is actuated to complete the control loop.
The method in accordance with the invention must not be confused with the balancing of a bridge such as has been mentioned in the fifth paragraph on page 4 of Published German Application 27 37 110. Such a balancing for a compensation of ambient capacitances, for instance, cannot be compared to a degenerative feedback control with storage of the last measured value.
In accordance with a desirable further feature of the invention the indicator comprises a limit comparator for receiving the output signal of the measuring device.